Altitude sickness occurs when a person ascends rapidly to a high elevation, typically above 8,000 feet, without allowing the body time to adjust to the reduced oxygen availability. Symptoms often include headache, nausea, fatigue, and dizziness. A common piece of advice given to travelers is to drink significant amounts of water to help prevent this condition. The relationship between drinking water and mitigating the effects of thin air is complex, involving several distinct biological mechanisms of fluid loss and acclimatization.
The Physiological Link: Altitude, Water Loss, and Dehydration
The body loses water more rapidly at high elevations due to several simultaneous factors. Low oxygen levels trigger hyperventilation, an immediate increase in breathing rate and depth. This increased ventilatory drive causes substantial insensible water loss as moisture is evaporated from the lungs into the typically dry mountain air. Respiratory water loss may increase by 3 to 4 percent for every 1,000 feet of ascent compared to sea level.
The air at high altitude often has very low humidity, which further accelerates fluid loss from the skin and respiratory tract. Simultaneously, the kidneys initiate high altitude diuresis as the body attempts to adjust its acid-base balance in response to the reduced oxygen. This physiological adjustment involves hormonal changes, causing the kidneys to excrete more water and sodium. This combination of increased breathing, dry air, and diuresis makes the risk of dehydration exceptionally high and can quickly deplete the body’s fluid stores.
Hydration’s Role in Preventing Acute Mountain Sickness
While proper fluid intake is a widely accepted practice for travelers, water alone does not prevent the onset of Acute Mountain Sickness. Acclimatization is the only true prevention. Dehydration, however, is a major contributor to the severity of AMS symptoms, including headaches and fatigue.
Maintaining adequate hydration is necessary for sustaining blood volume, which decreases rapidly upon acute exposure to altitude. When plasma volume drops, the blood thickens, making it more difficult for the heart to pump and transport oxygen efficiently. Preserving plasma volume helps the circulatory system deliver the limited available oxygen more effectively. Drinking only plain water, however, can be counterproductive due to fluid and sodium losses from altitude diuresis. If only water is consumed, sodium loss can lead to hyponatremia, meaning electrolytes are necessary for proper fluid absorption and balance.
Comprehensive Strategies for Altitude Acclimatization
To counteract the increased fluid loss at high elevations, travelers should aim to consume significantly more fluid than they would at sea level. A general recommendation suggests drinking an additional 1 to 1.5 liters of water daily when at altitude. A total fluid intake of approximately 3 to 4 liters per day is often necessary to maintain healthy hydration status, especially when physical activity is involved. Monitoring urine color—aiming for a clear or very pale yellow output—serves as a practical indicator of adequate hydration.
Hydration must be viewed as one component of a broader acclimatization plan.
Avoiding Diuretics
Fluids that impede acclimatization, such as alcohol and excessive caffeine, should be avoided, particularly during the initial days of ascent. Both substances act as diuretics, accelerating fluid loss. Alcohol can also suppress the respiratory drive, which is detrimental to acclimatization.
Rate of Ascent
The most important factor remains the rate of ascent. Travelers should avoid going too high too fast, ideally limiting the increase in sleeping elevation to no more than 1,600 feet per day once above 9,000 feet.
Nutrition
Proper nutrition, especially a diet high in carbohydrates, is beneficial as it can stimulate ventilation and improve oxygenation.